An Ohio man who raped and stabbed to death a pregnant woman became the first person executed with a previously untested two-drug cocktail on Thursday, and witnesses reported he gasped for air and took nearly 25 minutes to die.

Dennis McGuire — whose argument that the new protocol could cause terrifying and agonizing air starvation were rejected by a federal judge earlier in the week — was pronounced dead at 10:53 a.m.

An Associated Press reporter who witnessed the execution wrote that McGuire, 53, appeared to gasp several times and made several loud snorting or snoring sounds during a "prolonged" execution. Nearly 25 minutes passed between the time the lethal drugs began flowing and McGuire was pronounced dead.

His stomach rose and fell several times as he repeatedly opened and shut his mouth — his adult children sobbing from a few feet away as they watched, the AP reported.

Another witness, Columbus Dispatch writer Alan Johnson, reported McGuire "gasped loudly for air for about 13 minutes prior to his death," his chest heaving and his fist clenched. "Deep, rattling sounds emanated from his chest."

Executions under the old method were typically much shorter and did not cause the kind of sounds McGuire made.

Ohio adopted the new protocol — a combination of the sedative midozolam and the painkiller hydropmorphone — after the manufacturer of the old drug, pentobarbital, stopped selling it for lethal injections.

McGuire's lawyer, Allen Bohnert, called the execution a "failed experiment.'

"I would hope that somebody of some position of power up to or including the governor would realize the debacle that just occurred and have the intestinal fortitude to say, 'No more, this is going to be put on hold,'" Bohnert said.

Gov. John Kasich's office referred questions about the execution to the state correction's department, which said it planned to conduct the "action review" it does after each execution.

Richard Dieter, executive director of the Death Penalty Information Center, which opposes capital punishment, said the reports from the death chamber could lead the state or the courts to scrap the new method.

"This doesn’t sound like it was a complete disaster but they don’t want anything that even has the appearance of someone suffering or a delay in death being carried out," Dieter said. "This is going to be looked at with good concern.'

Dieter said it is not unheard of for an execution to take 15 minutes or even longer, but if the prisoner was physically struggling much of that time, it could be seen as cruel.

"This sounds like more discomfort than they would want in carrying out an execution," he said.

In a statement, the family of victim Joy Stewart noted that she suffered terror and pain in her last moments.

"We have forgiven him, but that does not negate the need for him to pay for his actions," they said.

McGuire was convicted of raping, sodomizing and slashing the throat of 22-year-old Stewart, who was eight months pregnant at the time of her murder in 1989.

In a bid for clemency last month, McGuire's lawyers argued that he had been mentally, physically and sexually abused as a child, which affected his brain development.

After the Parole Board turned him down, McGuire tried to get his execution halted on the grounds that the two-drug method might lead to a condition called "air hunger" before his actual death.

The court rejected the argument even as it acknowledged that his execution would be an "experiment" that could go wrong.

McGuire had also asked the governor to delay his date with death by offering to become an organ donor, but the request was turned down because he didn't identify any family members who could benefit.

The condemned prisoner who was supposed to be the first person executed with the new cocktail, child killer Ronald Phillips, won an eight-month reprieve by asking to donate a kidney to his mother and his heart to his sister. The state is studying the feasibility of that.

McGuire was moved to Ohio's death house on Wednesday and was given a last meal of roast beef, toasted bagel with cream cheese and onion, butter pecan ice cream, fried chicken, potato salad, fried potatoes with onion and Coca-Cola, officials said.

He was was awake all night talking on the phone and writing letters and had emotional visits with his son and daughter and other family members. He declined breakfast and a shower on Thursday morning.

Before he died, McGuire thanked his victim's family for their "kind words," apparently in a letter they sent.

"I'm going to heaven, I'll see you there when you come," he said to his children.

The Associated Press contributed to this report.

AP Photo/Kiichiro Sato, File

A 2005 photo of the death chamber at the Southern Ohio Correctional Facility in Lucasville, Ohio.